Lanthanide-crosslinked polymers for subterranean fluid containment

ABSTRACT

Polymers (and especially hydroxyethylcellulose) are crosslinked using a lanthanide as a crosslinking agent. The crosslinked polymers have utility in well completion, well stimulation, enhanced oil recovery, and subterranean fluid containment operations.

This application is a division of application Ser. No. 07/696,625, filedMay 7, 1991, now U.S. Pat. No. 5,111,886.

BACKGROUND

The present invention relates to (a) crosslinked polymers and (b)compositions and methods for (i) crosslinking polymers, (ii) gravelpacking well bores, (iii) fracturing subterranean formations, (iv)conforming the penetration depth of a subterranean acidizing procedure,(v) conforming the penetration depth of a subterranean caustic floodingprocedure, (vi) inhibiting the migration of a hazardous acid plume in asubterranean stratum, and (vii) reducing the water permeability ofsubterranean formations to improve the recovery of hydrocarbons.

Polymers are extensively used in well completion and enhanced oilrecovery procedures. In many of these procedures, e.g., fracturing,gravel packing, and subterranean formation permeability modificationoperations, it is preferred to employ crosslinked polymers.

Hydroxyethylcellulose (HEC) is very desirable for use in,;inter alia,fracturing and gravel packing operations because the polymer breakscleanly, i.e., does not cause formation damage or otherwise adverselyinterfere with the subterranean flow of hydrocarbons. However,heretofore, HEC has been virtually impossible to crosslink. To overcomethis problem, U.S. Pat. 4,552,215 and U.S. Pat. No. 4,553,601 describe aHEC crosslinking method that requires the additional cost of initiallychemically modifying the HEC by incorporating therein a pendant vicinaldihydroxy structure or a pendant aromatic polyol having at least twohydroxyl groups located on adjacent carbon atoms. The modified HEC isthen crosslinked using a crosslinking agent selected from titanium (IV),zirconium (IV), antimony (III), antimony (V), boron (III), lead (II),aluminum (III), arsenic (III), chromium (III), and niobium (V), whereinthe Roman numerals indicate the respective oxidation state of each ofthe foregoing ions.

SUMMARY OF THE INVENTION

The present invention provides a method for readily crosslinking HEC. Inone version of the invention, the crosslinking method entails raisingthe pH of an acidic solution (generally having a pH of less than about5) containing HEC and a lanthanide. Another version entails reacting HECand a sequestered lanthanide in an aqueous solution (typically having apH of about 5 to about 9) to form crosslinked HEC. In addition, acidicsolutions and aqueous solutions containing other polymers and alanthanide or a sequestered lanthanide are also crosslinkable usinganalogous procedures.

The solutions of the present invention are useful, for example, asfracturing and gravel packing fluids. When used as a fracturing fluid,the solutions preferably further contain a proppant. A particulate agentsuitable for use in forming a gravel pack is present in solutionsemployed as a gravel packing fluid.

Furthermore, the present invention includes procedures for conformingthe penetration depth of a subterranean formation acidizing operation.In one version, the penetration depth of a subterranean formationacidizing operation is conformed by sequentially injecting into theformation a slug of the polymer- and lanthanide-containing acidicsolution followed by a slug of an acidizing fluid. In another version, aslug of a crosslinkable-polymer-containing acidizing fluid and a slug ofa lanthanide-containing acidizing fluid are sequentially injected intothe subterranean formation.

Also encompassed by the present invention is a technique for conformingthe penetration depth of a subterranean formation caustic floodprocedure. In one embodiment, the penetration depth of the causticflooding procedure is conformed by sequentially injecting into theformation a slug of one of the solutions of the present invention, aspacer fluid slug, and a slug of a caustic flood fluid. In addition, thebreakthrough of a caustic flood at a producer well is inhibited orblocked by sequentially injecting into the formation through theproducer well a slug of a spacer fluid, a slug of one of the solutionsof the present invention, and another slug of the spacer fluid.

An additional aspect of the invention encompasses techniques forinhibiting the migration of a hazardous acid plume (such as leachatefrom a toxic landfill) through a subterranean stratum. One hazardousacid plume migration inhibition technique entails injecting into theacid plume a slug of one of the solutions of the present invention.Alternatively, the migration of a hazardous acid plume is inhibited byinjecting a slug of a solution of this invention into at least a portionof the subterranean formation not contacted by the acid plume but in themigration path of the acid plume.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment of the present invention, crosslinked polymers areformed by raising the pH of an acidic solution comprising a polymer anda lanthanide, while in another embodiment the crosslinking takes placeby mixing the polymer with a sequestered lanthanide. Exemplary polymerscrosslinkable by the foregoing processes include polyvinyl polymers,cellulose ethers, polysaccharides, lignosulfonates, ammonium saltsthereof, alkali metal salts thereof, as well as alkaline earth salts oflignosulfonates. Specific examples of typical water soluble polymers areacrylic acid-acrylamide copolymers, acrylic acid-methacrylamidecopolymers, polyacrylamides, polymethacrylamides, partially hydrolyzedpolyacrylamides, partially hydrolyzed polymethacrylamides, polyvinylalcohol, polyvinyl acetate, polyvinyl pyrrolidone, polyalkyleneoxides,carboxycelluloses, carboxyalkylhydroxyethyl celluloses, HEC, substitutedHEC, galactomannans (e.g., guar gum), substituted galactomannans (e.g.,hydroxypropyl guar (HPG)), heteropolysaccharides obtained by thefermentation of starch-derived sugar (e.g., xanthan gum), and ammoniumand alkali metal salts thereof. Because of its desirable properties whenused in well completion and enhanced oil recovery techniques, HEC is thepreferred polymer employed in the many of the well completion andenhanced oil recovery processes of this invention. In addition, as notedabove, heretofore it has been very difficult, if not impossible, tocrosslink HEC to form an aqueous-based gel.

The HEC employed in the present invention is preferably substantiallydevoid, more preferably virtually devoid, and most preferably totallydevoid of pendant vicinal dihydroxy groups, pendant aromatic polyolshaving at least two hydroxyl groups located on adjacent carbon atoms, orother pendant group not indigenous to HEC. As used in the specificationand claims, the term "substantially devoid" means that the degree ofsubstitution (DS) of the nonindigenous pendant groups on the HEC is lessthan about 0.01. The term "virtually devoid" means that the DS of thenonindigenous pendant groups on the HEC is less than about 0.001. Inaddition, the HEC typically has a molar substitution (MS) of about 1.5to about 3.5.

The polymer is generally present in the solutions of the presentinvention in a concentration of at least about 1,000 ppm. The preferredpolymer concentration of the solutions depends on the intended use ofthe respective solutions. For example, when intended for use inconforming the penetration depth of a subterranean formation acidizingprocedure, the acidic solution preferably contains about 1,000 to about10,000 ppm polymer, more preferably about 2,000 to about 9,000 ppmpolymer, and most preferably about 3,000 to about 8,000 ppm polymer.

However, when designed for use as fracturing and gravel packing fluidsas well as a fluid for conforming the penetration depth of a causticflood or inhibiting a caustic flood breakthrough at a production well,the acidic or aqueous solution preferably has a polymer concentration ofabout 2,500 to about 10,000 ppm (about 20 to about 85 pounds of polymerper 1,000 gallons of water), more preferably about 3,500 to about 7,500ppm (about 30 to about 60 pounds of polymer per 1,000 gallons of water),and most preferably about 4,000 to about 6,000 ppm (about 35 to about 50pounds of polymer per 1,000 gallons of water).

The molecular weight of the polymer is variable with the preferredranges being dependent on the specific polymer employed and the intendeduse of the respective solution of the invention. The general andpreferred polymer molecular weight ranges for the various polymers inthe various applications are similar, if not identical, to the molecularweight ranges employed by those skilled in the art in preparing fluids(such as fracturing fluids and gravel pack fluids) for correspondingapplications.

Regarding the term "lanthanide," as used in the specification andclaims, this term means the group of elements having atomic numbers 57through 71, namely, lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu),gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). The lanthanidesare used individually or admixed. Preferably, the lanthanide is selectedfrom the group consisting of La, Ce, and mixtures thereof, with La beingthe lanthanide of choice.

The lanthanide concentration in the solutions of the invention isusually at least about 10 ppm, preferably at least about 25 ppm, morepreferably at least about 50 ppm, and most preferably at least about 75ppm. Typically, about 100 to about 2,000 ppm of lanthanide is present inthe respective solution. Preferably, the lanthanide is present in theacidic and aqueous solutions in a concentration of about 100 to about1,000 ppm, more preferably about 120 to about 800 ppm, and mostpreferably about 150 to about 750 ppm.

The acidic solution generally has a pH of less than about 5. Thepreferred pH of the acidic solution also varies depending on theintended end use of the acidic solution. In the case of an acidicsolution intended for use in an acidizing procedure, the pH of thesolution is preferably less than about 3, more preferably less thanabout 2, and most preferably less than about 1. For fracturing, gravelpacking, and caustic flood applications, the pH of the acidic solutionis preferably about 1 to about 4, more preferably about 1 to about 3,and most preferably about 1 to about 2.

Strong acids are usually employed in forming the acidic solutions of thepresent invention. Exemplary strong mineral acids are hydrochloric acid,hydrofluoric acid, nitric acid, orthophosphoric acid, sulfurous acid andsulfuric acid. Common strong organic acids include, but are not limitedto, oxalic acid, formic acid, lactic acid, acetic acid, and citric acid.Mineral acids are the preferred strong acids.

The pH of the aqueous solution of this invention is typically greaterthan about 5. As in the case of the acidic solution, the preferred pH ofthe aqueous solution also varies depending on the intended end use ofthe aqueous solution. In general, the aqueous solution has a pH of about5 to about 9, preferably about 5.5 to about 8.5, and more preferablyabout 6.5 to about 7.5.

The solutions of the present invention optionally contain one or moreadditional ingredients such as gel breakers, sequestering agents,proppants suitable for use in hydraulically fracturing subterraneanformations, particulate agents suitable for use in forming a gravelpack, and corrosion inhibitors. Typical gel breakers include, but arenot limited to, enzymes (e.g., alpha and beta amylases,amyloglucosidase, invertase, maltase, cellulase, and hemicellulase) andfree radical generators (e.g., ammonium persulfate, potassiumdichromate, and potassium permanganate). The gel breaker concentrationsemployed in the solutions of the present invention are substantially thesame as employed by those skilled in the art in forming fluids forcorresponding end uses.

Exemplary sequestering agents are glutaric acid, glycolic acid,iminodiacetic acid, glucoheptonate, pentetic acid, citrate,nitrilotriacetic acid, edetic acid, adipic acid, and succinic acid.Iminodiacetic acid is the preferred sequestering agent.

The molar ratio of sequestering agent to lanthanide, when one or moresequestering agents are employed, is generally at least about 1:1. Thepreferred molar ratio depends on the desired polymer-lanthanide reactionrate. As a rule of thumb, the higher the sequestering agent-lanthanidemolar ratio, the slower the polymer-lanthanide reaction rate and thelonger it takes to form a gel. Usually, the preferred molar ratio is atleast about 2:1, with the more preferred molar ratio being at leastabout 3:1. Generally, the sequestering agent-lanthanide molar ratio isless than about 50:1.

While sequestering agents are optionally used in the polymer- andlanthanide-containing acidic solutions, their presence is very importantin the aqueous solutions of the present invention. Sequesteringagent-containing aqueous solutions, upon gelation, yield substantiallyuniform gels as opposed to pockets of gels in an aqueous environment.

Common proppants suitable for use in hydraulically fracturingsubterranean formations are quartz sand grains, tempered glass beads,sintered bauxite, resin coated sand, aluminum pellets, and nylonpellets. Generally, the proppants are employed in the solutions of thepresent invention intended for use as fracturing fluids and are used inconcentrations of roughly about 1 to about 10 pounds per gallon offracturing fluid. The proppant size is typically less than about 2 meshon the U.S. Sieve Series scale, with the exact size selected beingdependent on the particular type of formation to be fractured, theavailable pressure and pumping rates, as well as other factors known tothose skilled in the art.

Typical particulate agents employed in the solutions of the presentinvention used as gravel packing fluids include, but are not limited to,quartz sand grains, glass beads, synthetic resins, resin coated sand,walnut shells, and nylon pellets. The gravel pack particulate agents aregenerally used in concentrations of about I to about 20 pounds pergallon of gravel packing fluid. The size of the particulate agentemployed depends on the type of subterranean formation, the average sizeof formation particles, and other parameters known to those skilled inthe art. Generally, particulate agents of about 8 to about 70 mesh onthe U.S. Sieve Series scale are used.

One exemplary process for preparing the solutions of the presentinvention is as follows. Before mixing any chemicals into the wateremployed in making the solution, the water is generally filtered toprevent any suspended solids from damaging the formation by plugging thewellbore. Typically, the first chemicals added to the water are anyrequired salts (such as potassium or calcium chloride). The salts aregenerally employed to prevent clay problems in the formation and/or tohelp stabilize the resulting crosslinked polymer or gel.

In order for the polymer to experience a high degree of turbulenceduring the initial mixing stage, solid polymers and liquid-basedpolymers are usually added to the water through an eductor or a positivedisplacement pump, respectively. When desired, further mixing isachieved using either centrifugal pumps or other forms of agitation. Forgravel packing operations, the polymer solution is preferablyadditionally filtered through a diatomaceous earth filter and a finalpolish filter of about 1-5 micron rating.

Once the polymer is completely mixed into the water, the optionaladditives, when employed, are added to the mixing tank containing theaqueous polymer solution. Any solid additives are first dissolved inwater before they are added to the polymer solution.

Frequently, the last chemical added to the mixing tank is the lanthanidecrosslinking agent. When forming an acidic solution, the lanthanide isfirst dissolved in a strong acid (typically having a pH of less thanabout 1). In forming an acidic solution, a sufficient amount of one ormore sequestering agents is optionally also added to the strong acid toachieve the desired sequestering agent-lanthanide molar ratio. Asufficient amount of lanthanide-containing (or sequestering agent- andlanthanide-containing) strong acid composition is then added to thepolymer solution in the mixing tank to obtain an acidic solution havingthe desired pH and lanthanide concentration. The lanthanide-containing(or sequestering agent- and lanthanide-containing) strong acidcomposition is capable of being added to the polymer solution "on thefly," i.e., by introducing the composition into the polymer solution asthe solution is being transported through an injection line to the wellbore.

Exemplary lanthanides employed in forming the acidic solution arelanthanum oxide, lanthanum chloride, lanthanum carbonate octahydrate,lanthanum sulfate, lanthanum nitrate, ceric fluoride, ceric oxide, cericsulfate, cerous carbonate, cerous chloride, cerous iodide, cerousoxalate, cerous sulfate, praseodymium chloride, neodymium oxide,neodymium chloride, neodymium sulfate, samarium sulfate, europicchloride, europic sulfate, gadolinium chloride, gadolinium sulfate,gadolinium nitrate, terybium chloride hexahydrate, dysprosium nitrate,erbium oxide, erbium chloride, erbium sulfate, thulium oxide, thuliumchloride heptahydrate, ytterbium oxide, and lutetium sulfate.

The acidic solution forms a gel when the pH of the solution exceeds acertain minimum threshold gelling pH. The minimum threshold gelling pHof a specific acidic solution is dependent on the particular lanthanideemployed therein. For example, when the lanthanide is lanthanum, thecrosslinking reaction commences at a pH of about 2, and when thecrosslinking agent is cerium, the polymer begins to crosslink at aboutpH 4. It is preferred to increase the pH about one or more pH unitsabove the crosslinking pH threshold to ensure gelation. The pH of theacidic solution is increased in the formation by the acidic solutionmixing with indigenous subterranean water and/or by being spent byreacting with minerals present in the formation. When desired, the pH ofthe acidic solution is also raised by releasing a base, e.g., throughcoiled tubing, at a preselected depth in the well bore.

To prepare an aqueous solution of the present invention, the lanthanideand a sufficient amount of one or more sequestering agents are firstdissolved in water to achieve the desired sequestering agent-lanthanidemolar ratio. An adequate amount of the resulting sequestering agent- andlanthanide-containing composition is then added to the polymer solutionin the mixing tank to obtain an aqueous solution having the desiredlanthanide concentration.

The pH of the aqueous solution is adjusted, when necessary, to thedesired level by the addition of an acid or base. Preferably, the pH isadjusted using a dilute acid or base, e.g., dilute hydrochloric acid ordilute sodium hydroxide, respectively.

The lanthanide employed in forming the aqueous solution of the presentinvention must be soluble in a aqueous medium. Such water solublelanthanides include, but are not limited to, lanthanum chloride,lanthanum sulfate, lanthanum nitrate, cerous chloride, cerous iodide,cerous sulfate, praseodymium chloride, neodymium oxide, neodymiumchloride, neodymium sulfate, samarium sulfate, europic chloride, europicsulfate, gadolinium chloride, gadolinium sulfate, gadolinium nitrate,terbium chloride hexahydrate, dysprosium nitrate, erbium oxide, erbiumchloride, erbium sulfate, thulium chloride heptahydrate, and lutetiumsulfate.

The aqueous solution of the present invention forms a gel by the polymercompeting with the sequestering agent for the lanthanide. Accordingly,the higher the sequestering agent-lanthanide molar ratio, the longer theonset of gelation is delayed.

Another optional additive employed in formulating the aqueous solutionsof the present invention are base precursors. The more widely known baseprecursor classes are ammonium salts, quaternary ammonium salts, urea,substituted ureas, coordinated compounds, and salts of a strong base anda weak acid, with the preferred base precursors being urea, thiourea,ammonium chloride, and mixtures thereof. When employed, the baseprecursors are used in a concentration sufficient to raise the pH of theaqueous solution at least about a 0.5 pH unit, more preferably at leastabout 1 pH unit, and most preferably about 2 or more pH units.

The solutions of the present invention are obtained when all the desiredingredients have been mixed together. The resulting acidic or aqueoussolution is then often injected into a subterranean formation through awell bore (e.g., a production or injection well) that penetrates atleast a portion of the formation. The specific conditions and procedurefor injecting the solutions of this invention are determined by theintended use of each solution. Some of these parameters are well knownto those skilled in the art. For example, when an acidic or aqueoussolution is employed as a gravel packing fluid it is typically injectedinto the formation in accordance with the procedure discussed in U.S.Pat. No. 4,552,215, this patent being incorporated herein in itsentirety by reference. The aqueous solution of the present invention ispreferably used as a gravel packing fluid.

The acidic and aqueous solutions are equally preferred for use asfracturing fluids and are usually injected into the formation usingprocedures analogous to those disclosed in U.S. Pat. No. 4,488,975, U.S.Pat. No. 4,553,601, Howard et al., Hydraulic Fracturing, Society ofPetroleum Engineers of the American Institute of Mining, Metallurgical,and Petroleum Engineers, Inc., New York, N.Y. (1970), and chapter 8 ofAllen et al., Production Operations, Well completions, Workover, andStimulation, 3rd edition, volume 2, Oil & Gas Consultants International,Inc., Tulsa, Okla. (1989) (Allen), these publications being incorporatedherein in their entirety by reference.

When injected into a subterranean formation as a fracturing fluid, it isdesirable to use a base to ensure that gelation commences as soon as theinjected fluid leaves the well bore. The base is preferably introducedor injected into the well bore at a depth sufficiently close to the endof the well bore for the pH of the acidic solution to rise above theminimum crosslinking pH threshold as the solution leaves the well bore.Generally, the base is injected as a liquid (e.g., ammonia or sodiumhydroxide) as a or gas (e.g., ammonia gas) through tubing having anoutlet at the desired injection depth.

When employed in an acidizing well stimulation procedure, the acidicsolution of the present invention is preferably used. The acidicsolution is injected into the subterranean formation usually after aninitial injection of an acidizing fluid. Generally, a minimum of about500 gallons of acidizing fluid are injected during the course of anentire acidizing treatment. Typically, up to approximately 30 volumepercent of the entire injected acidizing fluid consists of the acidicsolution of this invention. Preferably, the acidic solution comprisesabout 1 to about 25 volume percent of the acidizing fluid, morepreferably about 5 to about 20 percent of the acidizing fluid, and mostpreferably about 10 to about 15 volume percent of the acidizing fluid.

The acidic solution is injected as a single slug or as a plurality ofalternating slugs, with the intervening slugs being the acidizing fluid.

Aside from using the acidic solution of the present invention and theabove described details particular to such usage, the acidizingprocedure remains otherwise virtually unchanged. Details for conductingacidizing treatments are extensively discussed in Williams et,al.,Acidizing Fundamentals, Society of Petroleum Engineers of the AmericanInstitute of Mining, Metallurgical, and Petroleum Engineers, Inc., NewYork, N.Y. (1979) and in chapter 7 of Allen, these publications beingincorporated in their entirety by reference.

When injected into a subterranean formation during the course of anacidizing treatment, the acidic solution of the present invention tendsto enter the more water permeable portions of the formation. As theacidic solution is diluted by mixing with indigenous subterranean waterand/or spent by reacting with materials present in the formation, the pHof the solution rises and the polymer begins to crosslink once thecrosslinking pH threshold is crossed. The resulting gel acts to divertany subsequently injected acidizing fluid into less water permeableportions of the formation, thereby increasing the efficiency of theacidizing treatment.

In an alternative acidizing treatment embodying features of the presentinvention, the acidizing treatment is conducted by preferablysequentially injecting into at least a portion of the formation apolymer-containing acidizing fluid that is generally substantiallydevoid of any lanthanide, a slug of a lanthanide-containing acidizingfluid, and a slug of just acidizing fluid. In this version of theinvention, the polymer-containing acidizing fluid is preferablyvirtually devoid and more preferably totally devoid of any lanthanide.When used in conjunction with the polymer-containing acidizing fluid,the term "substantially devoid" means that the polymer-containingacidizing fluid contains less than about 0.1 ppm lanthanide and the term"virtually devoid" means that the polymer-containing acidizing fluidcontains less than about 0.01 ppm lanthanide.

While the amount of the polymer and the lanthanide employed in thislatter version of the invention are similar to the amounts used in thepreviously discussed acidizing treatment embodiment, the concentrationsof the polymer and the lanthanide are approximately doubled in therespective slugs in which they are present in this version of theinvention.

The polymer-containing acidizing fluid also preferably enters the morewater permeable zones of the formation. When the lanthanide reaches thepolymer and the pH of the fluid is raised by mixing with indigenoussubterranean water and/or depleting the acid due to the acidizing fluidreacting with materials present in the formation, the lanthanidecrosslinks the polymer as discussed above with similar beneficialresults.

To avoid any permanent or long lasting permeability damage, the polymerselected for use in the acidic solution employed in an acidizingtreatment (as well as in gravel packing and hydraulic fracturingoperations) is one that preferably readily degrades at formationtemperature and/or by the action of a gel breaker. HEC, HPG (especiallyfor fracturing operations), and xanthan (especially for gravel packingprocedures) are exemplary of such degradable polymers--with HEC beingthe most preferred because it breaks very cleanly.

However, when the acidic or aqueous solutions of the present inventionare employed in conjunction with caustic flooding, caustic breakthroughprevention, and acid plume containment operations, it is preferred touse polymers that do not readily degrade at formation temperature.Exemplary preferred polymers for these procedures are polyacrylamide andcopolymers of acrylamide and (a) acrylic acid, (b) vinyl pyrrolidone,(c) sodium 2-acrylamido-2-methylpropane sulfonate (sodium AMPS®), and(d) mixtures of (a), (b), and (c).

Usually, the viscosity average molecular weight of the polyacrylamideand acrylamide copolymers employed in these solutions is generally lessthan about 20,000,000 and preferably less than about 15,000,000. In themajority of cases, the polyacrylamide and acrylamide copolymer have aviscosity average molecular weight of at least about 1,000,000.Preferably, the viscosity average molecular weight of the polyacrylamideand acrylamide copolymer is at least about 5,000,000.

A relationship exists between the minimum desired polyacrylamide and/oracrylamide copolymer concentration, the preferred polyacrylamide and/oracrylamide copolymer concentration, and the viscosity average molecularweight of the polyacrylamide and/or acrylamide copolymer employed in thesolutions used in these embodiments of the invention. In general, theconcentration of polyacrylamide and acrylamide copolymer required toform a visible gel increases with decreasing molecular weight. Thisrelationship is illustrated for polyacrylamides and acrylamidecopolymers of various viscosity average molecular weights in thefollowing Table A.

                  TABLE A                                                         ______________________________________                                                   Minimum Desired                                                                              Preferred                                                      Polyacrylamide Polyacrylamide                                                 Or Acrylamide Co-                                                                            Or Acrylamide Co-                                   Viscosity Average                                                                        polymer Concentra-                                                                           polymer Concentra-                                  Molecular Weight                                                                         tion, ppm by weight                                                                          tion, ppm by weight                                 ______________________________________                                        >7,000,000 1,000          2,000-6,000                                           3,000,000-                                                                             1,500          3,000-8,000                                          7,000,000                                                                      1,000,000-                                                                             2,000          4,000-10,000                                        <3,000,000                                                                    <1,000,000 7,000          9,000-30,000                                        ______________________________________                                    

In an exemplary procedure for conforming the penetration depth of acaustic flood, the formation in question is first pretreated with a slugof one of the solutions of the present invention. Typically, thisinitial slug consists of about 500 to about 2,000 barrels. To avoidimmediately crosslinking the polymer present in the solutions of theinvention, the caustic flood is separated from the initial slug by aspacer fluid, e.g., water. The volume of spacer fluid used is roughlyabout one-half the volume employed in the initial slug.

When injected into a subterranean formation during the course of acaustic flood, the solutions of the present invention tend to enter themore water permeable portions of the formation. The solutions of thepresent invention gel either (a) as the acidic solution is diluted bymixing with indigenous subterranean water and/or spent by reacting withmaterials present in the formation, (b) as the polymer competes with thesequestering agent for the lanthanide, and/or (c) when the caustic floodcontacts the solution and raises its pH above the crosslinking pHthreshold. The resulting gel acts to divert any subsequently injectedcaustic flood fluids into less water permeable portions of theformation, thereby increasing the efficiency of the caustic floodtreatment.

Because a caustic flood tends to be conducted for a substantial lengthof time, a slug of one or more of the solutions of the present inventionis periodically injected into the formation to further help ensure theuniformity of the caustic flooding treatment. The volume of solution ofthis invention injected in these subsequent slugs also is about 500 toabout 2,000 gallons per slug. In addition, the subsequent slug ofsolution of this invention is sandwiched between two spacer slugs (e.g.,slugs of water) to avoid premature gelling. The volume of each of thesesandwiching spacer fluid slugs is approximately half the volume of thecorresponding sandwiched slug of this invention.

Outside of the details involved in using the solutions of the presentinvention in a caustic flood, the remaining portion of the caustic floodprocedure is conducted using techniques well known to those skilled inthe art. Exemplary caustic flooding techniques are discussed in vanPoollen et al., Fundamentals of Enhanced Oil Recovery, PennWell Books,Tulsa, Okla. (1980), chapter 6, this publication being incorporatedherein in its entirety by reference.

Besides being used to conform the penetration depth of a caustic floodoperation, the solutions of the present invention are also useable toinhibit or block a caustic flood breakthrough at a production well. Inthis latter version, when a caustic flood breakthrough occurs or isimminent at a production well, a slug of a spacer fluid, such as oil orother hydrocarbon, is first injected into, the formation through theproduction well. The spacer fluid volume is roughly about 500 to about1,000 barrels. After injecting the spacer fluid, about 250 to about 500barrels of one of the solutions of the present invention, and preferablythe acidic solution, is injected into the formation through theproduction well. A slug of about 500 to about 1,00 barrels of anotheroil or other hydrocarbon spacer fluid is employed to displace theinjected solution of the present invention from the well bore and pushit into the formation. When the caustic flood fluid contacts thesolution of this invention, the lanthanide crosslinks the polymer toform a gel. The resulting gel inhibits, and preferably blocks, thecaustic flood fluid from reaching the production well.

An interesting aspect of the caustic flooding embodiments of the presentinvention is that most prior art polymer crosslinking systems areincapable of gelling or, if already gelled, are not stable at the highpH employed in caustic flooding operations. In contrast, the solutionsof the present invention are capable of gelling, and the resulting gelsare stable, at the high pH used in caustic flooding procedures.

The solutions of the present invention are also useful for inhibitingthe migration of a hazardous acid plume through a subterranean stratumof a landfill or natural subterranean formation. In one version, theacidic solution is injected through one or more well bores, e.g.,observation wells, into the acid plume. The volume of acidic solutioninjected per well bore depends on the volume and cross-sectional area ofthe plume intersecting the well bore. In rough terms, about 100 to about10,000 barrels of the acidic solution are injected per well. The pH ofthe acidic solution is generally less than or equal to the pH of theacid plume. The lanthanide selected for use in the acidic solution isone that is soluble at the pH of the acidic solution. The concentrationof the lanthanide in the acidic solution used in this version of theinvention is the same as noted above and the concentration of thepolymer is generally at least about 1,000 ppm, preferably about 1,000 toabout 10,000 ppm, more preferably about 2,000 to about 9,000 ppm, andmost preferably about 3,000 to about 8,000 ppm.

Once in the acid plume, the acidic solution commingles with the plume,with the resulting composition gelling when the pH of the compositionrises above the crosslinking pH threshold. The migration of the acidplume is inhibited by the presence of the gel in the formation orstratum.

In another version, one of the solutions of the present invention isinjected through a well bore into a subterranean region or stratumoutside the acid plume but in the migration path of the plume. Theaqueous solution is preferably used in this version of the invention.The concentration of polymer and lanthanide employed in the aqueoussolution is the same as used in the acidic solution employed in theembodiment of the invention discussed in the preceding paragraph. Themolar ratio of sequestering agent to lanthanide selected depends on thelength of time desired to delay the onset of gelation--the higher themolar ratio, the longer the delay. Generally, the molar ratio is atleast about 3:1.

When an acidic solution is injected into the subterranean formation orstratum outside the acid plume, the composition of the solution isgenerally the same as when the acidic solution is injected into the acidplume. However, the pH of the acidic solution is selected to permit thedesired volume of the acidic solution to be injected into the formationprior to forming a gel. In general, the volume of the acidic solutioninjectable into the formation prior to forming a gel is inverselyproportional to the pH of the acidic solution, i.e., the lower the pH ofthe injected acidic solution, the greater the volume of acidic solutioninjectable into the formation prior to gelation. Accordingly, in thisversion of the invention, the pH of the injected acidic solution ispreferably less than 1. Whatever the pH of the acidic solution, a geleventually forms when the pH of the acidic solution rises above thecrosslinking pH threshold by mixing with indigenous subterranean waterand/or the acid being spent by reacting with indigenous formationmaterials.

When injecting either the acidic solution or the aqueous solution intothe formation outside the acid plume, the injected volume of thesolution is also roughly about 100 to about 10,000 barrels. In addition,the resulting gel obtained by injecting either the aqueous solution orthe acidic solution in the migration path of the acid plum inhibits theflow of the plume past the gel blockage.

EXAMPLES

The following examples--which are for purposes of illustrating and notlimiting the invention--demonstrate the formation and stability ofvarious gels.

EXAMPLES 1-4 Crosslinking Polyacrylamide

Polyacrylamide was crosslinked in accordance with the followingprocedure. Lanthanum oxide and ceric sulfate were dissolved in separatesolutions of about 10 weight percent sulfuric acid. Thelanthanum-containing solution contained about 3,837 ppm La and thecerium-containing solution contained about 3,586 ppm Ce. Both solutionshad a pH of about 0.

Pusher 700E brand polyacrylamide (available from Dow Chemical Co.) wasdissolved in water. The resulting solution contained about 4,666 ppmpolyacrylamide and had a pH of about 7.

An aliquot (about 1 ml) of one of the lanthanide-containing solutionswas mixed with an aliquot (about 14 ml) of the polyacrylamide-containingsolution to yield an acidic solution having a pH of about 0. Four suchacidic samples were prepared, with two of the acidic samples containinglanthanum and the other two containing cerium. The cerium and lanthanumconcentrations in the respective samples are set forth below in Table I.

Ammonia gas was then bubbled into each of the four acidic samples. Inone case a sufficient amount of ammonia was bubbled in to raise the pHof the sample to about 1.5 and the condition of the sample was examined.In another portion of the experiment, the ammonia gas was bubbled inuntil the sample completely gelled. (As used in the specification andclaims, the terms "completely gelled," "gel completely formed," and"complete gel" mean that additional bubbling of ammonia gas into thesample does not further increase the gel quality of the sample.) The pHof the completely gelled sample and the quality of the resulting gelwere then determined. All the results noted in this paragraph are alsolisted in the following Table I.

                  TABLE I                                                         ______________________________________                                                        Concentration                                                      Crosslinking                                                                             In Acidic Solu-                                                                           Final                                             Ex.  Agent      tion, ppm   pH     Appearance                                 ______________________________________                                        1    Ce         239         1.5    No gelation yet                            2    Ce         239         12     5.sup.a                                    3    La         255         1.5    No gelation yet                            4    La         255         11     5-                                         ______________________________________                                         .sup.a The samples evaluated in Examples 1-4 of Table I and throughout th     remaining examples were visually rated according to the Gel Quality Ratin     Key set forth in the following Table II.                                 

                  TABLE II                                                        ______________________________________                                        Gel Quality Rating Key                                                        ______________________________________                                        5                Rigid Gel                                                    4                Elastic Gel                                                  3                Weak Gel                                                     2                Viscous Fluid                                                1                Water-like Fluid                                             ______________________________________                                    

The results shown in Table I indicate that cerium and lanthanum, twoexemplary lanthanides, are capable of crosslinking polyacrylamide, anexemplary crosslinkable polymer, to form very good gels.

EXAMPLES 5-16 Crosslinking Hydroxypropyl Guar (HPG)

This series of examples demonstrates that HPG crosslinks when alanthanide is used as a crosslinking agent. In particular, tap water wasmixed with distilled water to obtain stock water comprising about 35volume percent tap water. A stock lanthanum concentrate was obtained bymixing about 5 ml concentrated hydrochloric acid (about 37.5 weightpercent hydrochloric acid) and about 94.023 ml of the stock water andthen adding about 0.977 g lanthanum oxide to the resulting solution. Thestock lanthanum concentrate contained about 3,750 ppm lanthanum. Similarprocedures were used to obtain stock glutaric acid sequestered lanthanumand stock iminodiacetic acid sequestered lanthanum concentrates. Inparticular, the stock glutaric acid sequestered lanthanum concentratewas formed by mixing about 5 ml concentrated hydrochloric acid withabout 91.863 ml stock water. To the resulting solution were sequentiallyadded about 0.977 g lanthanum and about 2.16 g glutaric acid. For thestock iminodiacetic acid sequestered concentrate, about 5 mlconcentrated hydrochloric acid was mixed with about 91.823 ml stockwater and about 0.977 g lanthanum and about 2.2 g iminodiacetic acidwere sequentially added to the resulting solution. Both sequesteredlanthanum concentrates contained about 3,750 ppm lanthanum and had asequestering agent-lanthanum molar ratio of about 6:1. A stock HPGsolution containing about 10,000 ppm HPG was prepared by adding asufficient amount of HPG-102 brand HPG (obtained from Aqualon Co.) to analiquot of the stock water. As shown below in Table III, varying amountsof the stock lanthanum or stock sequestered lanthanum concentrate werecombined with about 12 ml of the stock HPG solution and a sufficientamount of the stock water to yield about 15 ml of each listed acidicsolution. The final lanthanum concentration in each acidic solution isalso noted in Table III.

The resulting sequestered or unsequestered lanthanum- and HPG-containingacidic solutions were allowed to sit at room temperature andperiodically visually examined. On the twelfth day ammonia gas wasbubbled into each sample. The results of this experiment are shown inthe following Table III.

                  TABLE III                                                       ______________________________________                                                  Elapsed Time, Days                                                       Lanthanum  1          11       12.sup.a                                  Ex.  ml     ppm     pH   Rating                                                                              pH   Rating                                                                              pH   Rating                         ______________________________________                                        Unsequestered Lanthanum                                                        5   0.5    125     1.7  1+    1.6  1+    10   5                               6   1.0    250     1.5  1+    1.5  1+    10   5                               7   2.0    500     1.3  1+    1.2  1+    10   4+                              8   3.0    750     1.3  1     1.1  1+    10   4+                             Glutaric Acid Sequestered Lanthanum                                            9   0.5    125     4.7  1+    4.6  1+    10   4-                             10   1.0    250     4.4  1+    4.4  1+    10   5                              11   2.0    500     4.4  1+    4.1  1+    10   4+                             12   3.0    750     4.2  1+    4.3  1+    10   4+                             Iminodiacetic Acid Sequestered Lanthanum                                      13   0.5    125     5.4  1+    5.1  1+    10   4+                             14   1.0    250     3.9  1+    4.1  1+    10   4+                             15   2.0    500     3.4  1+    3.5  1+    10   2-                             16   3.0    750     3.4  1+    3.0  1+    9.5  2-                             ______________________________________                                         .sup.a The acidic solutions were bubbled with ammonia gas on day 12.     

The results shown in the above Table III indicate that no gelling tookplace in either sequestered or unsequestered lanthanum containingsamples until the pH was raised on day 12.

EXAMPLES 17-36 Determination of Optimum Lanthanide and PolymerConcentrations

The following experiments were conducted to determine the optimum HECand lanthanum concentrations for forming gels. A stock potassiumchloride solution was prepared by dissolving a sufficient quantity ofpotassium chloride in distilled water for the resulting solution tocontain about 2 weight percent potassium chloride. To make a stock HECsolution, a sufficient amount of Natrosol 250 HHR brand HEC(manufactured by Hercules Inc. and having a MS of about 2.5) wasdissolved in an aliquot of the potassium chloride stock solution for theHEC stock solution to contain about 10,000 ppm HEC. A stock lanthanumconcentrate was made by mixing about 5 ml concentrated hydrochloric acidand about 94.023 ml of the stock potassium chloride solution and thenadding about 0.977 g lanthanum oxide to the resulting solution. Thestock lanthanum concentrate contained about 3,750 ppm lanthanum.

Various amounts of the lanthanum stock concentrate were mixed with thedifferent amounts of the stock HEC and potassium chloride solutions toform the samples listed in Table IV below. The initial pH of each acidicsolution was taken and then ammonia gas was bubbled in until a gelcompletely formed. The initial pH, the pH when gelation was complete,and the gel ratings over time for each sample are recorded in thefollowing Table IV.

                  TABLE IV                                                        ______________________________________                                             HEC,    La,    Initial                                                                             Final                                                                              Days                                           Ex.  ppm     ppm    pH    pH   1     2    3   6                               ______________________________________                                        17   2500    125    1.8   11.2 2+    2+   2   2-                              18   2500    250    .sup. N/T.sup.a                                                                     11.0 2-    2-   2+  2-                              19   2500    500    N/T   10.8 2-    2-   2+  2-                              20   2500    750    1.5   10.5 2-    2-   2   2-                              21   3000    125    2.2   10.5 3-    3-   3-  3-                              22   3000    250    1.7   10.5 3+    3+   3-  3-                              23   3000    500    1.5   10.0 3+    3+   3-  3-                              24   3000    750    1.3   10.5 3-    3-   2-  2-                              25   4000    125    2.2   10.7 4+    3+   3-  3-                              26   4000    250    2.0   10.5 4+    3+   3-  3-                              27   4000    500    1.5   10.0 4+    4-   3-  2+                              28   4000    750    1.3   10.2 3-    3-   3-  2+                              29   5000    125    2.2   10.5 4+    4+   4+  4-                              30   5000    250    N/T   10.5 4+    4++  4++ 4+                              31   5000    500    N/T    9.6 4+    4+   4++ 4+                              32   5000    750    1.5    9.7 4+    4+   4+  4-                              33   7500    125    2.2   10.2 4+    4+   4++ 4+                              34   7500    250    N/T   10.0 4++   5-   5-  5-                              35   7500    500    N/T   10.0 4++   4++  5-  5-                              36   7500    750    1.5    9.6 4+    4++  5-  5-                              ______________________________________                                         .sup.a N/T means not taken.                                              

The data set forth in the above Table IV indicate that the best gels areprepared when the HEC concentration in the solution is at least about5,000 ppm.

EXAMPLES 37-42 Effect of Gel Breakers on Gel Stability

In this group of experiments the effect of an enzyme and a free radicalgenerator on the quality and stability of gels was determined. The stockpotassium chloride solution, the stock HEC solution, and the stocklanthanum solution described in Examples 17-36 were also used in thepresent Examples 37-42. In addition, a stock HPG solution containingabout 10,000 ppm HPG was prepared by dissolving a sufficient quantity ofHPG-102 brand HPG in an aliquot of the stock potassium chloridesolution. A stock enzyme gel breaker solution containing about 1 weightpercent enzyme was prepared by mixing a sufficient quantity of an enzymegel breaker in an aliquot of the potassium chloride stock solution. Inaddition, a stock persulfate solution containing about 1 weight percentsodium persulfate was prepared by mixing enough sodium persulfate withanother aliquot of the stock potassium chloride solution.

An aliquot of the HEC or HPG stock solution was mixed with an aliquot ofthe lanthanum stock solution and, except for the controls, with about a100 μl aliquot of either the enzyme or persulfate gel breaker stocksolution to form the samples listed below in Table V. The samples werebubbled with sufficient ammonia gas to form a complete gel or for a twominute period, whichever came first. The initial pH of each sample, thepH of each sample after bubbling, the initial gel rating, and the gelrating after the samples were stored for four days at room temperatureare set forth in the following Table V.

                  TABLE V                                                         ______________________________________                                                                              Gel Rating,                                  HEC,    Gel       La,  pH        Day                                     Ex.  ppm     Breaker   ppm  Initial                                                                             Final 1     4                               ______________________________________                                        37   8000    Enzyme    500  1.5   10.3  3-    1                               38   8000    Persulfate                                                                              500  1.5   10.5  1     1                               39   8000    None      500  1.3   10.5  5+    5-                              ______________________________________                                                                              Gel Rating,                                  HPG,    Gel       La,  pH        Day                                     Ex.  ppm     Breaker   ppm  Initial                                                                             Final 1     4                               ______________________________________                                        40   8000    Enzyme    500  1.5   10.3  2-    1                               41   8000    Persulfate                                                                              500  1.5    8.8  4++   1                               42   8000    None      500  1.3   10.0  5+    5-                              ______________________________________                                    

The gel ratings noted in the above Table V indicate that the addition ofa gel breaker when forming an acidic solution of the present inventionreduces the stability of the gel. In addition, the gel quality rating of1 obtained in Examples 37-38 and 40-41 shows that HEC and HPG gels breakvery cleanly, even at room temperature, when an enzyme and sodiumpersulfate gel breakers are used.

EXAMPLES 43-63 Effect of Post Gelation Added Gel Breakers on GelStability

In this set of experiments the effect of adding an enzyme after gelationon the stability of a gel was investigated. The stock potassium chloridesolution, the stock HEC solution, and the stock lanthanum concentratedescribed in Examples 17-36, and the stock enzyme gel breaker solutionprepared in Examples 37-42 were also used in the present Examples 43-63.

A series of aliquots of the HEC stock solution were mixed with acorresponding series of aliquots of the lanthanum stock solution andammonia was bubbled through each resulting sample to form complete gels.After gelation, about 100 μl of the 1 weight percent enzyme stocksolution was added to each gel, except for the controls. Prior to addingthe stock enzyme solution to the gels, each sample was stored for abouttwo days at room temperature. After the addition of the enzyme stocksolution, all the enzyme-containing gels and the controls were stored atthe various temperatures noted in the following Table VI. Table VI alsolists the gel rating of each gel during the course of these experiments.

                                      TABLE VI                                    __________________________________________________________________________                    Enzyme                                                                             Storage                                                  HEC,   La,                                                                              Day,  Added                                                                              Temp.                                                                              Hours,                                              Ex.                                                                              ppm ppm                                                                              1  2  Here °F.                                                                         1  2  3  4  21                                      __________________________________________________________________________    43 4545                                                                              227                                                                              4+ 4+ No   100  3+ 3- 3- 3- 3-                                      44 4545                                                                              454                                                                              4+ 4+ No   100  3  3- 3- 3- 2+                                      45 4545                                                                              227                                                                              4+ 4+ Yes   75  3- 3- 3- 3- 3-                                      46 4545                                                                              454                                                                              4+ 4+ Yes   75  3- 3- 3- 2+ 2-                                      47 4545                                                                              227                                                                              5- 4+ Yes  100  3- 2- 1+ 1+ 1                                       48 4545                                                                              454                                                                              4+ 4+ Yes  100  2- 2- 1+ 1+ 1                                       49 4545                                                                              227                                                                              4+ 4+ Yes  125  1  1  1  1  1                                       50 4545                                                                              454                                                                              4+ 3- Yes  125  1+ 1  1  1  1                                       51 2500                                                                              125                                                                              2- 2- Yes  160  2- 2- 2- 2- 1                                       52 2500                                                                              250                                                                              2- 2- Yes  160  2- 2- 2- 2- 1                                       53 2500                                                                              500                                                                              2- 2- Yes  160  2- 2- 2- 2- 1+                                      54 2500                                                                              750                                                                              2- 2- Yes  160  1+ 1  1  1  1                                       55 5000                                                                              125                                                                              4- 4- Yes  160  3- 2+ 2- 1+ 1                                       56 5000                                                                              250                                                                              4+ 4+ Yes  160  3- 2- 2- 1+ 1                                       57 5000                                                                              500                                                                              4+ 4+ Yes  160  3- 2- 2- 2- 1                                       58 5000                                                                              750                                                                              4- 4- Yes  160  2- 2- 2- 2- 1                                       59 7500                                                                              125                                                                              4+ 4+ Yes  160  3  3- 2- 1  1                                       60 7500                                                                              250                                                                              5- 5- Yes  160  3+ 3- 2- 1+ 1                                       61 7500                                                                              500                                                                              5- 5- Yes  160  3- 2+ 2+ 2- 1                                       62 7500                                                                              750                                                                              5- 5- Yes  160  3- 2+ 2+ 2- 1                                       63 8000                                                                              500                                                                              5+ 5+ No   160  5- 5- 5- 5- 1                                       __________________________________________________________________________

The data in the above Table VI indicate that the post gelation additionof an enzyme gel breaker degrades gels, the effect being moresignificant at higher temperatures. In addition, the results of thecontrol samples shown in Table VI demonstrate that HEC gels self-degradeat about 100° F. or greater.

EXAMPLES 64-75 Effect of Temperature and Post Gelation Addition of GelBreakers on Gel Stability

In this group of experiments the effect of temperature as well as anenzyme and a free radical generator on gel stability was determined. Thestock HEC solution and the stock lanthanum concentrate employed in 17-36were used to formulate acidic solution samples containing about 5,000ppm HEC and about 500 ppm lanthanum. Complete gels were formed bybubbling ammonia gas through each sample. About 100 μl of the enzyme orpersulfate gel breaker stock solution of Examples 37-42, when employed,was added after forming the complete gel. The initial pH of each acidicsolution was about 1.3 and the pH at which complete gelation occurredranged between about 9.5 and about 10. The initial gel ratings, storagetemperatures, and gel ratings during storage are shown in the followingTable VII.

                                      TABLE VII                                   __________________________________________________________________________             Storage                                                              Gel      Temp.,                                                                             Hours Stored,                                                   Ex.                                                                              Breaker                                                                             °F.                                                                         0  1  2  3  4  5  6  8  24                                      __________________________________________________________________________    64 None   75  3+ 2+ 2+ 2+ 2+ 2  2  2  2-                                      65 None  100  3+ 4+ 4- 3+ 3- 3- 4- 2+ 2-                                      66 None  125  3+ 2- 2- 2- 2- 2- 2- 1+ 1+                                      67 None  160  3+ 2- 2- 1+ 1+ 1+ 1+ 1  1                                       68 Enzyme                                                                               75  3+ 2- 2- 2- 1+ 1+ 1+ 1+ 1+                                      69 Enzyme                                                                              100  3+ 1+ 1+ 1+ 1+ 1+ 1+ 1  1                                       70 Enzyme                                                                              125  3+ 1+ 1  1  1  1  1  1  1                                       71 Enzyme                                                                              160  3+ 1  1  1  1  1  1  1  1                                       72 Persulfate                                                                           75  3+ 2- 2- 2- 1+ 1+ 1+ 1+ 1+                                      73 Persulfate                                                                          100  3+ 1+ 1+ 1+ 1+ 1+ 1+ 1+ 1                                       74 Persulfate                                                                          125  3+ 2- 1+ 1+ 1+ 1+ 1  1  1                                       75 Persulfate                                                                          160  3+ 1  1  1  1  1  1  1  1                                       __________________________________________________________________________

The results set forth in the above Table VII indicate that thedegradation of HEC gels by the enzyme gel breaker and the sodiumpersulfate gel breaker is comparable.

EXAMPLES 76-83 Effect of Temperature and Pregelation Addition of GelBreakers on Gel Stability

In these experiments the procedure employed in formulating the gels ofExamples 64-75 was repeated with one modification, namely, the gelbreaker, when used, was added to the sample prior to gelation. The gelswere stored at various temperatures for about 20 hours. At about the20th hour, all the samples were stored at about 115° F. The results ofthis group of experiments are summarized in the following Table VIII.

                                      TABLE VIII                                  __________________________________________________________________________             Storage              Storage                                         Gel      Temp.,                                                                             Hours,          Temp.,                                                                             Hours,                                     Ex.                                                                              Breaker                                                                             °F.                                                                         0  1   2  18 20 °F.                                                                         1  3                                       __________________________________________________________________________    76 None   75  4+ 4+  4+ 4- 4- 115  2- 1+                                      77 None  100  4+ 4+  4- 2  2  115  1+ 1+                                      78 None  125  4+ 4-  3  1+ 1  115  1  1                                       79 None  130  4+ 3+  3- 1+ 1  115  1  1                                       80 Enzyme                                                                               75  4- 3++ 3+ 3- 3  115  1+ 1+                                      81 Enzyme                                                                              100  4- 4-  3- 2- 1+ 115  1+ 1+                                      82 Persulfate                                                                           75  4  3++ 3  4  3- 115  1+ 1+                                      83 Persulfate                                                                          100  4  4-  3- 2- 2- 115  1+ 1+                                      __________________________________________________________________________

The results shown in above Table VIII reinforce the conclusions drawnfrom the data set forth in Table VII, supra. In addition, the datalisted in Table VIII indicate that the presence of a gel breaker doesnot prevent the formation of good gels.

EXAMPLES 84-86 Effect of Sequestering Agents on the Gelation of HPG

To determine the effect of sequestering agents on the gelation of HPG,an aliquot (about 3 ml) of the stock sequestered lanthanum concentratedescribed in Examples 9-16 was mixed with an aliquot (about 6 ml) of thestock HPG solution and an aliquot (about 6 ml) of the stock water ofExamples 5-16. In the case of the control, an aliquot (about 3 ml) ofthe stock lanthanum concentrate of Examples 5-8 was employed in place ofthe stock sequestered lanthanum concentrate. Each of the samplescontained about 8,000 ppm HPG and about 750 ppm lanthanum. Table IX setsforth the initial gel ratings, the initial pHs, the gel rating after thesamples were bubbled with ammonia for a sufficient time to raise thesample pH to about 9, and the gel rating about 24 hours after theammonia bubbling was stopped.

                  TABLE IX                                                        ______________________________________                                                                     Gel Rating                                                Initial Gel Rating  24 Hours                                              Sequester-                                                                              Gel         After Bubbling                                                                          After Bubbling                           Ex.  ing Agent Rating  pH  With Ammonia                                                                            With Ammonia                             ______________________________________                                        84   None      1+      1.2 5+        5+                                       85   Glutaric  1+      3.6 3+         4++                                          Acid                                                                     86   Iminodi-  1+      3.1 1+        5-                                            acetic Acid                                                              ______________________________________                                    

The results of Table IX indicate that HPG gels in the presence of asequestering agent. In addition, as shown in Example 86, thesequestering agent occasionally acts to delay the onset of gelation.

EXAMPLES 87-88 Effect of Sequestering Agent on Gelation of HEC

To determine the effect of a sequestering agent on the gelation of HEC,a stock HEC solution containing about 10,000 ppm HEC was prepared byadding a sufficient amount of Natrosol 250 brand HEC to an aliquot ofthe stock water of Examples 5-16. An aliquot (about 3 ml) of theiminodiacetic stock sequestered lanthanum concentrate described inExamples I3-16 was mixed with an aliquot (about 6 ml) of the stock HECsolution and an aliquot (about 6 ml) of the stock water of Examples5-16. In the case of the control, an aliquot (about 3 ml) of the stocklanthanum concentrate of Examples 5-8 was employed in place of the stocksequestered lanthanum concentrate. Each of the samples contained about8,000 ppm HEC and about 750 ppm lanthanum. Table X sets forth theinitial pHs, the initial gel ratings, the pHs when either a complete gelformed or at about two minutes after ammonia bubbling commenced(whichever came first), and the gel ratings over a period of time whilestoring the gels at about 75° F.

                                      TABLE X                                     __________________________________________________________________________              Bubbling With Ammonia                                               Sequestering                                                                            Before  After                                                                             Gel Rating Days                                         Ex.                                                                              Agent  Gel Rating                                                                          pH                                                                              pH  1  2   3  6   7                                         __________________________________________________________________________    87 None   1+    1.5                                                                              9.6                                                                              4+ 4++ 5- 5-  5-                                        88 Iminodiacetic                                                                        1+    3.5                                                                             10.5                                                                              1+ 3-  3- 4++ 4++                                          Acid                                                                       __________________________________________________________________________

The results of Table X indicate that a HEC gels in the presence of asequestering agent. In addition, Example 88 shows that the sequesteringagent iminodiacetic acid acts to delay the onset of gelation.

Although the present invention has been described in considerable detailwith reference to some preferred versions, other versions are possible.For example, the solutions of the present invention are employable inthe selective subterranean water permeability modification techniquesdisclosed in U.S. Pat. No. 5,145,012 incorporated herein in its entiretyby reference. Polyacrylamides are also the preferred polymers for thesolutions of the present invention when used in those selectivesubterranean water permeability modification techniques. Therefore, thespirit and scope of the appended claims should not necessarily belimited to the description of the preferred versions contained herein.

What is claimed is:
 1. A method for inhibiting the migration of ahazardous acid plume through a subterranean stratum, the methodcomprising the step of injecting into at least a portion of the acidplume a slug of a composition capable of forming a gel, the compositioncomprising:(a) a crosslinkable polymer; and (b) a lanthanide.
 2. Themethod of claim 1 wherein the crosslinkable polymer is selected from thegroup consisting of polyvinyl polymers, cellulose ethers,polysaccharides, lignosulfonates, ammonium salts thereof, alkali metalsalts thereof, and alkaline earth salts of lignosulfonates.
 3. Themethod of claim 1 wherein the crosslinkable polymer is selected from thegroup consisting of acrylic acid-acrylamide copolymers, acrylicacid-methacrylamide copolymers, polyacrylamides, polymethacrylamides,partially hydrolyzed polyacrylamides, partially hydrolyzedpolymethacrylamides, polyvinyl alcohol, polyvinyl acetate, polyvinylpyrrolidone, polyalkyleneoxides, carboxyalkylhydroxyethyl celluloses,hydroxyethylcellulose, galactomannans, hydroxypropyl guar,heteropolysaccharides obtained by the fermentation of starch-derivedsugar, and ammonium and alkali metal salts thereof.
 4. The method ofclaim 1 wherein the crosslinkable polymer is selected from the groupconsisting of polyacrylamide and copolymers of acrylamide and (A)acrylic acid, (B) vinyl pyrrolidone, (C) sodium2-acrylamido-2-methylpropane sulfonate, and(D) mixtures of (A), (B), and(C).
 5. The method of claim 1 wherein the crosslinkable polymer ispresent in the composition in a concentration of at least about 1,000ppm.
 6. The method of claim 1 wherein the crosslinkable polymer ispresent in the composition in a concentration of about 1,000 to about10,000 ppm.
 7. The method of claim 1 wherein the lanthanide is selectedfrom the group consisting of lanthanum, cerium, and mixtures thereof. 8.The method of claim 1 wherein the lanthanide concentration in thecomposition is at least about 10 ppm.
 9. The method of claim 1 whereinthe composition has a pH less than about
 5. 10. A method for inhibitingthe migration of a hazardous acid plume through a subterraneanformation, the method comprising the steps of:(A) injecting a slug of acomposition capable of forming a gel into at least a portion of thesubterranean formation not contacted by the acid plume but in themigration path of the acid plume, the composition comprising:(a) acrosslinkable polymer; and (b) a lanthanide; and (B) allowing the gel toinhibit the migration of the acid plume.
 11. The method of claim 10wherein the composition has a pH greater than about
 5. 12. The method ofclaim 10 wherein the composition further comprises a sequestering agent.13. The method of claim 10 wherein the composition has a pH greater thanabout 5 and further comprises a sequestering agent.
 14. The method ofclaim 10 wherein the composition further comprises a sequestering agentand the molar ratio of the sequestering agent to lanthanide in thecomposition is at least about 1:1.
 15. The method of claim 10 whereinthe composition has a pH less than about
 5. 16. The method of claim 10wherein the crosslinkable polymer is selected from the group consistingof polyvinyl polymers, cellulose ethers, polysaccharides,lignosulfonates, ammonium salts thereof, alkali metal salts thereof, andalkaline earth salts of lignosulfonates.
 17. The method of claim 10wherein the crosslinkable polymer is selected from the group consistingof acrylic acid-acrylamide copolymers, acrylic acid-methacrylamidecopolymers, polyacrylamides, polymethacrylamides, partially hydrolyzedpolyacrylamides, partially hydrolyzed polymethacrylamides, polyvinylalcohol, polyvinyl acetate, polyvinyl pyrrolidone, polyalkyleneoxides,carboxyalkylhydroxyethyl celluloses, hydroxyethylcellulose,galactomannans, hydroxypropyl guar, heteropolysaccharides obtained bythe fermentation of starch-derived sugar, and ammonium and alkali metalsalts thereof.
 18. The method of claim 10 wherein the crosslinkablepolymer is selected from the group consisting of polyacrylamide andcopolymers of acrylamide and (A) acrylic acid, (B) vinyl pyrrolidone,(C) sodium 2-acrylamido-2-methylpropane sulfonate, and (D) mixtures of(A), (B), and (C).
 19. A system for inhibiting the migration of ahazardous acid plume, the system comprising:(a) an acid plume present inat least a portion of a subterranean stratum; (b) a well borepenetrating at least a portion of the subterranean stratum; and (c) acomposition present in at least a portion of the well bore, thecomposition comprising:(a) a crosslinkable polymer; and (b) alanthanide.